Vehicle upgrade bug and "phantom" upgrade order.

I was sent a save file containing a bug so I could report it.
A player decided to upgrade some excavators; one of them was sent to a vehicle factory and vanished, yet the upgrade order remained active. The player couldn't figure out what had happened, so I decided to investigate the issue.
It turns out that when an old excavator is scrapped to make way for an advanced version, another vehicle factory claims the newly available slot in the vehicle depot to produce a truck. Consequently, the old excavator is scrapped, but the new one cannot be released due to the depot's vehicle limit. The excavators will only leave the factory once a slot opens up in the depot.
The problem is that the player doesn't understand why the excavator isn't being released from the factory; there is no notification indicating that the vehicle depot has reached its capacity limit.
It would probably be best to prioritize vehicle upgrades so that the slot isn't taken by new vehicles being produced at a different factory.
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